Fluidized bed combustion (FBC) is a combustion technology used in power plants, primarily to burn solid fuels. FBC power plants are more flexible than conventional power plants in that they can be fired on coal, coal waste or biomass, among other fuels. The term FBC covers a range of fluidized bed processes, including circulating fluidized bed (CFB) boilers, bubbling fluidized bed (BFB) boilers and other variations thereof. In an FBC power plant, fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process in a combustor, causing a tumbling action which results in turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides a means for more effective chemical reactions and heat transfer in the combustor.
During the combustion process of fuels which have a sulfur-containing constituent, e.g., coal, sulfur is oxidized to form primarily gaseous SO2. In particular, FBC reduces the amount of sulfur emitted in the form of SO2 by a desulfurization process. A suitable sorbent, such as limestone containing CaCO3, for example, is used to absorb SO2 from flue gas during the combustion process. In order to promote both combustion of the fuel and the capture of sulfur, FBC power plants operate at temperatures lower than conventional combustion plants. Specifically, FBC power plants typically operate in a range between about 850° C. and about 900° C. Since this allows coal to combust at cooler temperatures, NOx production during combustion is lower than in other coal combustion processes.
To further increase utilization of the fuel and efficiency of sulfur capture, a cyclone separator is typically used to separate solids, e.g., unutilized fuel and/or limestone, entrained in flue gas leaving the combustor. The separated solids are then returned to the combustor via a recirculation means, e.g., a recirculation pipe, to be used again in the combustion process. A sealpot, sometimes referred to as a “j-leg,” maintains a seal between the combustor and the cyclone separator to prevent unwanted escape of flue gas from the combustor backward, e.g., in a direction opposite to flow of the separated solids into the combustor, through the recirculation pipe.
Air systems in an FBC power plant are designed to perform many functions. For example, air is used to fluidize the bed solids consisting of fuel, fuel ash and sorbent, and to sufficiently mix the bed solids with air to promote combustion, heat transfer and reduce emissions (e.g., SO2, CO, NOx and N2O). In order to accomplish these functions, the air system is configured to inject air, designated primary air (PA) or secondary air (SA), at various locations and at specific velocities and quantities.
In addition, fluidizing air and transport air are typically supplied to the sealpot to facilitate flow of solids and gas through the sealpot, as shown in FIG. 1. Referring to FIG. 1, a sealpot 10 of the prior art includes a downcomer standpipe 15, a fluidizing/transport bed 20, a fluidizing air source 25, a discharge standpipe 30, a transport air source 35 and a weir 40 separating the fluidizing/transport bed 20 and the discharge standpipe 30. The fluidizing/transport bed 20 includes a fluidizing zone supplied with fluidizing air from the fluidizing air source 25, and a transport zone supplied with transport air from the transport air source 35. The fluidizing air source 25 and the transport air source 35 may be separate components, as shown in FIG. 1, or, alternatively, the fluidizing air source 25 and the transport air source 35 may be combined as a single air source (not shown).
As shown in FIG. 1, in the sealpot 10 of the prior art, solids from the combustion process flow downward from the cyclone separator (not shown) through the downcomer standpipe 15 to the fluidizing/transport bed 20. The solids are fluidized by the fluidizing air from the fluidizing air source 25 and/or the transport air source 35 in the fluidizing zone of the fluidizing/transport bed 20. The fluidized solids are then transported through the transport zone of the fluidizing/transport bed 20 to the discharge standpipe 30 by the fluidizing air from the fluidizing air source 25 and/or the transport air supplied from the transport air source 35, thereby forming an expansion bed in the discharge standpipe 25. More specifically, solids which are transported above the weir 40, e.g., above a weir height Hweir, form the expansion bed, thereby causing some solids to flow over the weir 40 into the discharge pipe 30. In addition, some gases, primarily fluidizing air from the fluidizing air source 25 and transport air from the transport air source 35, flow to the combustor via the discharge standpipe 30. Thus, the sealpot 10 forms a seal, thereby preventing flue gases in the combustor from flowing backward through the sealpot 10, e.g., upward through the downcomer standpipe 15 back into the cyclone (105 shown in FIG. 4).
In the sealpot 10 of the prior art, it is difficult to control a size of the expansion bed due to the nature of unsteady solid/gas interactions, particularly during transition of operations and resulting changes in gas and solids flow rate to the combustor (not shown) through the discharge standpipe 30. As a result, an excessive amount of solids flow over the weir 40, e.g., the size of the sealpot expansion bed suddenly becomes excessively large, which may disrupt the distribution of the fluidization air at the downstream combustor. In such a case, oscillation of pressure changes may occur in the system.
In addition, a range of flow rates of solids regulation through the sealpot 10 is limited in the sealpot 10 of the prior art, since the size of the expansion bed cannot be precisely regulated to control a number of different flow rates of solids over the weir. Put another way, solids are essentially either flowing over the weir or they are not; there are no precisely defined discrete flow rates and different flow rates are therefore difficult to establish a steady continuous flow, especially during transition of operations, as described above.
Accordingly, it is desired to develop a sealpot and a method for controlling a flow rate of solids through the sealpot, such that the sealpot has benefits including, but not limited to: increased solids flow control range and accuracy of regulation thereof; increased steady state seal maintainability; decreased flue gas escape; decreased solids sudden overflow; and increased turndown ratio of solids flow control using a sealpot.